Aqueous polymer dispersions

ABSTRACT

Aqueous polymer dispersions containing polymers obtainable by free-radical polymerization of unsaturated monomers, and sugared starch, and the use thereof.

The present invention relates to aqueous polymer dispersions of polymersobtainable by free-radical polymerization of unsaturated monomers whichcontain at least one added starch-degradation product which isobtainable by hydrolysis in the aqueous phase and has a weight averagemolecular weight M_(w) of from 2500 to 25000. The present inventionfurthermore relates to a process for the preparation of aqueous polymerdispersions of this type and to the use of these dispersions.

Aqueous polymer dispersions are systems which contain, as the dispersephase, polymer particles dispersed in an aqueous dispersion medium.Since the dispersed polymer particles tend to agglomerate for reasons ofthermodynamic stability, aqueous polymer dispersions are generallystabilized using surfactants.

Such surfactants which have been recommended include starches and starchderivatives, which, compared with other surfactants, are advantageousinasmuch as they are renewable raw materials. The use of non-degradedstarches or derivatives of non-degraded starches which imply nodegradation is disadvantageous compared with the use of degradedstarches or derivatives thereof inasmuch as the first-mentioned are notentirely satisfactory with respect to their solubility in water and withrespect to their rheological behavior in aqueous media.

DE-A 3 922 784 relates to a process for the preparation of aqueouspolymer dispersions by free-radical polymerization of unsaturatedmonomers in the presence of unmodified or chemically modified dextrins,where the polymers must contain at least 20% by weight of a diene, andthe proportion by weight of dextrins having a molecular weight ofgreater than 5000 is at least 50% by weight and the proportion by weightof dextrins having a molecular weight of greater than 100,000 is at most5% by weight. For the purposes of this invention, dextrins are degradedstarches whose degradation is effected by heating with or withoutaddition of chemicals, it being possible to recombine degradationfragments under the degradation conditions to form new bonds which werenot present in this form in the original starch. Since the term dextrinis sometimes also used as a general term for all high-molecular-weightfragments of starches, the term roast dextrins, which is conventional inthe specialist literature (see, for example, Gunther Tegge, Starke undStarkederivate, Behr's Verlag, Hamburg 1984, p. 173), will be used herefor the dextrins of DE-A 3 922 784. Roast dextrins are prepared byheating moist-dry starch, usually in the presence of small amounts ofacid. The most important reaction during the roasting is limitedhydrolytic degradation of the starch, which, due to the spatial density,is accompanied by recombination and branching reactions.

Examples of typical roast dextrins are the commercially available whiteand yellow dextrins, and furthermore dextrins marketed under the tradenames Noredux® and Tackidex® (Gunther Tegge, Starke und Starkederivate,Behr's Verlag, Hamburg 1984, p. 173-177).

The process of DE-A 3 922 784 has the disadvantage that on the one handit is restricted to certain monomers and on the other hand uses roastdextrins which must be prepared by the roasting process, which isrelatively complex, in particular with respect to the necessaryreactors.

EP-A 334 515 relates to aqueous dispersions of polyacrylates which areobtainable by the free-radical polymerization of the monomers in thepresence of a dextrin of which at least 70% has a molecular weight inthe range from 1000 to 25000 and not more than 10% has a molecularweight above 25000. These aqueous dispersions have the disadvantage thatthe dextrins which must be used according to EP-A 334 515 are againroast dextrins which are only obtainable by a relatively complex processand which even then do not give aqueous polymer dispersions which arelong-term stable with respect to their dynamic viscosity if the roastdextrins are employed in pregelled form. Thus, the dynamic viscositydoubles within 5 days after preparation in the most favorable workingexample.

DE-A 3 323 804 relates to aqueous polymer dispersions which areobtainable, inter alia, by free-radical aqueous emulsion polymerizationin the presence of "starches degraded by acid hydrolysis and frequentlyalso known as dextrins" and are particularly suitable for thepreparation of polymer powders. The preparation of the dextrins by acidhydrolysis is not described in greater detail in this publication sinceit is alleged to be known to a person skilled in the art. However, theworking examples indicate that DE-A 3 323 804 also relates to roastdextrins having said disadvantageous properties. For example, thedextrins employed included yellow and white dextrins.

EP-A 276 770 discloses aqueous polymer dispersions obtainable bypolymerization of certain unsaturated monomers in the presence ofdegraded starches, where the starches are characterized by theirintrinsic viscosity. These polymer dispersions have the disadvantagethat the identity of the degraded starches used as examples iscompletely open and the viscosity is a rather undefined measure fortheir characterization, as shown in EP-A 334 515.

It is an object of the present invention to provide aqueous polymerdispersions of polymers obtainable by free-radical polymerization ofunsaturated monomers, whose disperse distribution is stabilized,compared with the prior art, with the aid of readily accessible degradedstarches and which simultaneously, in an entirely satisfactory manner,

a) are obtainable in a simple manner,

b) have increased stability under mechanical or thermal load,

c) have increased stability toward addition of electrolyte,

d) are not restricted to polymers of specific monomers,

e) have increased flowability,

f) have a flowability which remains essentially unchanged, even overlong periods,

g) are low-foaming,

h) can be obtained with a mean particle diameter which is variable overa broad range,

i) have a reduced content of microflocculations (specks) andmacroflocculations (coagulate),

j) do not discolor on extended storage,

k) are suitable for the preparation of redispersible powders

l) do not undergo any phase separation even on extended storage.

We have found that this object is achieved by the aqueous polymerdispersions defined at the outset. Starch degradation products having aweight average molecular weight of from 2500 to 25000 and obtainable byhydrolysis in the aqueous phase are usually known as sugared starches,in contrast to roast dextrins, and are commercially available as such(for example the C* PUR Products 01906, 01908, 01910, 01912, 01915,01921, 01924, 01932 and 01934 from Cerestar Deutschland GmbH, D-1150Krefeld 12).

Sugared starches of this type differ chemically from roast dextrinsthrough the fact, inter alia, that recombination and branching areessentially impossible, evident not least in different molecular weightdistributions, on hydrolytic degradation in an aqueous medium (usuallysuspensions or solutions), which is generally carried out at solidscontents of from 10 to 30% by weight and preferably with acid or enzymecatalysis. Thus, sugared starches which have a bimodal molecular weightdistribution have proven particularly advantageous according to theinvention.

The preparation of sugared starches is generally known and is described,inter alia, in Gunther Tegge, Starke und Starkederivate, Behr's Verlag,Hamburg 1984, p. 173 and p. 220 ff. and in EP-A 441 197. The sugaredstarches to be used according to the invention are preferably thosewhose weight average molecular weight M_(w) is in the range from 4000 to16000, particularly preferably in the range from 6500 to 13000.

The sugared starches to be used according to the invention are normallycompletely soluble in water at room temperature, the solubility limitgenerally being above 50% by weight, which proves particularlyadvantageous for the preparation of the aqueous polymer dispersionsaccording to the invention.

It has furthermore proven favorable for the sugared starches to be usedaccording to the invention to have a nonuniformity U (defined as theratio between the weight average weight M_(w) and the number averagemolecular weight M_(n) ; U characterizes the molecular weightdistribution) in the range from 6 to 12. U is particularlyadvantageously from 7 to 11 and very particularly advantageously from 8to 10.

It is furthermore advantageous for the proportion by weight of thesugared starches having a molecular weight of below 1000 to be usedaccording to the invention to be at least 10% by weight, but not morethan 70% by weight. This proportion by weight is particularly preferablyin the range from 20 to 40% by weight.

It is furthermore advisable to use sugared starches to be used accordingto the invention whose dextrose equivalent DE is from 5 to 40,preferably from 10 to 30, particularly preferably from 10 to 20. The DEvalue characterizes the reduction capacity, relative to the reductioncapacity of anhydrous dextrose, and is determined in accordance with DIN10308, Edition 5.71, produced by the German Standards Committee onFoodstuffs and Agricultural Products (cf. also Gunther Tegge, Starke undStarkederivate, Behr's Verlag, Hamburg 1984, p. 305).

It has also been found that aqueous polymer dispersions which have aparticularly favorable property profile are obtained if sugared starchesto be used according to the invention whose 40% strength by weightaqueous solutions have a dynamic viscosity η⁴⁰ [Pa.s], determined inaccordance with DIN 53 019 at 25° C. and a shear gradient of 75 s⁻¹, offrom 0.01 to 0.06, preferably from 0.015 to 0.04, particularlypreferably from 0.02 to 0.035.

It should be noted at this point that molecular weight data for sugaredstarches to be used according to the invention are based in thispublication, unless expressly stated otherwise, on determinations bymeans of gel permeation chromatography, carried out under the followingconditions:

Columns: 3 steel units measuring 7.5×600 mm, filled with TSK gel G 2000PW; G 3000 PW and G 4000 PW. Mesh 5 μm

Eluent: Distilled water.

Temp.: RT (room temperature)

Detection: Differential refractometer (for example ERC 7511)

Flow rate: 0.8 ml/min, pump (for example ERC 64.00)

Injection 20 μl, valve ( for example VICI 6-way vol.: valve)

Evaluation: Bruker Chromstar GPC software

Calibration: The calibration was carried out in the low-molecular-weightrange using glucose, raffinose, maltose and maltopentose. For thehigher-molecular-weight range, pullulan standard having a polydispersity<1.2 was used.

The starting starches for the preparation of the sugared starches to beused according to the invention can in principle be any native starches,such as cereal starches (e.g. corn, wheat, rice or barley), tuber androot starches (e.g. potatoes, tapioca roots or arrowroot) or sagostarches.

An essential advantage of the sugared starches to be used according tothe invention is that they can be used without any further chemicalmodification, apart from the extremely simple partial hydrolysis of thestarting starch for their preparation. However, it is of course alsopossible to use them according to the invention in chemically modifiedform, for example by etherification or esterification. This chemicalmodification may also have been carried out in advance on the startingstarch before its degradation. Esterifications are possible using bothinorganic and organic acids, or anhydrides or chlorides thereof.Phosphated and acetylated degraded starches are of particular interest.The most common method of etherification is treatment with organohalogencompounds, epoxides or sulfates in aqueous alkaline solution.Particularly suitable ethers are alkyl ethers, hydroxyalkyl ethers,carboxyalkyl ethers and allylethers. It is also possible to use productsof the reaction with 2,3-epoxypropyltrimethylammoniumchloride.Chemically unmodified sugared starches are preferred.

Suitable monomers which can be polymerized by means of free radicalsinclude, in particular, monoethylenically unsaturated monomers, such asolefins, e.g. ethylene, vinylaromatic monomers, such as styrene,α-methylstyrene, o-chlorostyrene and vinyltoluenes, vinyl and vinylidenehalides, such as vinyl chloride and vinylidene chloride, esters madefrom vinyl alcohol and monocarboxylic acids having 1 to 18 carbon atoms,such as vinyl acetate, vinyl propionate, vinyl n-butyrate, vinyl laurateand vinyl stearate, esters made from α,β-monoethylenically unsaturatedmono- and dicarboxylic acids, preferably having 3 to 6 carbon atoms,such as, in particular, acrylic acid, methacrylic acid, maleic acid,fumaric acid and itaconic acid, with alkanols generally having from 1 to12, preferably from 1 to 8, in particular from 1 to 4, carbon atoms,such as, in particular, ethyl, n-butyl, isobutyl and 2-ethylhexylacrylates and methacrylates, dimethyl maleate and n-butyl maleate,nitriles of α,β-monoethylenically unsaturated carboxylic acids, such asacrylonitrile, and C₄₋₈ conjugated dienes, such as 1,3-butadiene andisoprene. Said monomers are essentially insoluble in the aqueous mediaand generally form the principal monomers, which normally make up aproportion of greater than 50% by weight, based on the total amount ofmonomers to be polymerized. Monomers which, when polymerized alone,usually give homopolymers of increased water solubility are normallyonly copolymerized as modifying monomers,in amounts, based on the totalamount of monomers to be polymerized, of less than 50% by weight, ingeneral from 0.5 to 20% by weight, preferably from 1 to 10% by weight.

Examples of such monomers are α,β-monoethylenically unsaturated mono-and dicarboxylic acids having 3 to 6 carbon atoms, and amides thereof,e.g. acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconicacid, acylamide and methacrylamide, furthermore vinylsulfonic acid andwater-soluble salts thereof, and N-vinylpyrrolidone. Monomers whichusually increase the internal strength of films formed by the aqueouspolymer dispersion are generally likewise only copolymerized in minoramounts, usually from 0.5 to 10% by weight, based on the total amount ofmonomers to be polymerized. Such monomers normally contain an epoxide,hydroxyl, N-methylol or carbonyl group or at least two non-conjugatedethylenically unsaturated double bonds. Examples of this areN-alkylolamides of α,β-monoethylenically unsaturated carboxylic acidshaving 3 to 10 carbon atoms and esters thereof with alcohols having 1 to4 carbon atoms, of which N-methylolacrylamide andN-methylolmethacrylamide are very particularly preferred, monomerscontaining two vinyl radicals, monomers containing two vinylideneradicals, and monomers containing two alkenyl radicals. Particularlysuitable here are diesters of dihydric alcohols withα,βmonoethylenically unsaturated monocarboxylic acids, of which in turnacrylic acid and methacrylic acid are preferably employed. Examples ofsuch monomers containing two non-conjugated ethylenically unsaturateddouble bonds are alkylene glycol diacrylates and dimethacrylates, suchas ethylene glycol diacrylate, 1,3-butylene glycol diacrylate,1,4-butylene glycol diacrylate and propylene glycol diacrylate,divinylbenzene, vinyl methacrylate, vinyl acrylate, allyl methacrylate,allyl acrylate, diallyl maleate, diallyl fumarate andmethylenebisacrylamide. In addition to monomers containing unsaturateddouble bonds, it is also possible to copolymerize minor amounts, usuallyfrom 0.01 to 4% by weight, based on the monomers to be polymerized, ofmolecular weight regulators, such as tert-dodecyl mercaptan. Suchsubstances are preferably added to the polymerization zone in a mixturewith the monomers to be polymerized.

Preferred classes of aqueous polymer dispersions according to theinvention are those whose polymers are obtainable by free-radicalpolymerization of monomer mixtures which comprise

from 50 to 100% by weight of esters of acrylic and/or methacrylic acidwith alkanols having 1 to 12 carbon atoms and/or styrene (class I) or

from 70 to 100% by weight of styrene and/or butadiene (class II) or

from 70 to 100% by weight of vinyl chloride and/or vinylidene chloride(class III), class I being particularly preferred and preferablycovering the following monomer compositions:

90 to 99% by weight of esters and acrylic and/or methacrylic acid withalkanols having 1 to 8 carbon atoms and/or styrene, and

1 to 10% by weight of acrylic acid, methacrylic acid or a mixturethereof.

Of very particular interest are the following monomer compositions:

from 90 to 99% by weight of n-butyl acrylate and/or styrene,

from 1 to 10% by weight of acrylic acid and/or methacrylic acid.

The sugared starches to be used according to the invention may be usedeither as the only dispersant or as a mixture with other surfactants. Ifthey are the only dispersants employed, they are normally present in theaqueous monomer dispersion according to the invention in an amount offrom 1 to 120% by weight, based on the amount of polymerized monomers.

Suitable secondary surfactants are in principle the protective colloidsand emulsifiers otherwise usually employed as dispersants. A detaileddescription of suitable protective colloids is given in Houben-Weyl,Methoden der organischen Chemie, Volume XIV/1, Makromolekulare Stoffe,Georg-Thieme-Verlag, Stuttgart, 1961, pp. 411 to 420. Suitable secondaryemulsifiers are anionic, cationic and nonionic emulsifiers. It ispreferred for the secondary surfactants to be exclusively emulsifiers,whose relative molecular weights, in contrast to the protectivecolloids, are usually below 2000. It is of course necessary, if mixturesof surfactants are used, for the individual components to be compatiblewith one another, which can be checked in case of doubt by means of afew preliminary experiments. The secondary surfactants used arepreferably anionic and nonionic emulsifiers. Examples of customarysecondary emulsifiers are ethoxylated fatty alcohols (degree ofethoxylation from 3 to 50, C₈ - to C₃₆ -alkyl radical), ethoxylatedmono-, di- and trialkylphenols (degree of ethoxylation from 3 to 50,C₄ - to C₉ -alkyl radicals), alkali metal salts of dialkylesters ofsulfosuccinic acid and alkali metal and ammonium salts of alkyl sulfates(C₈ - to C₁₂ -alkyl radical), of ethoxylated alkanols (degree ofethoxylation from 4 to 30, C₁₂ - to C₁₈ -alkyl radical), of ethoxylatedalkylphenols (degree of ethoxylation from 3 to 50, C₄ - to C₉ -alkylradical), of alkylsulfonic acids (C₁₂ - to C₁₈ -alkyl radical) and ofalkylarylsulfonic acids (C₉ - to C₁₈ -alkyl radical). Further suitableemulsifiers are given in Houben-Weyl, Methoden der organischen Chemie,Volume XIV/1, Makromolekulare Stoffe, Georg Thieme Verlag, Stuttgart,1961, pages 192 to 208. If secondary surfactants are used (in general inamounts of up to 5% by weight, based on the amount of polymerizedmonomers), aqueous polymer dispersions according to the invention havealso proven advantageous if the proportion of sugared starches to beused according to the invention is 0.01% by weight, based on the amountof polymerized monomers. A particularly advantageous factor, inter alia,is the entirely satisfactory biodegradability of the dispersantsaccording to the invention, which is presumably attributable not leastto the fact that essentially no repolymerization with formation of bondsdifferent from those in the natural starting starch takes place duringthe starch degradation for their preparation.

The aqueous polymer dispersions according to the invention arepreferably prepared by polymerizing the monomers by the free-radicalaqueous emulsion polymerization process in the presence of the sugaredstarches to be used according to the invention. The emulsionpolymerization temperature is generally from 30° to 95° C., preferablyfrom 75° to 90° C. The polymerization medium may either comprise wateralone or a mixture of water and water-miscible liquids, such asmethanol. It is preferred to use water alone. The emulsionpolymerization can be carried out either as a batch process or in theform of a feed process, including a step or gradient procedure.Preference is given to the feed process, in which part of thepolymerization batch is heated to the polymerization temperature andpartially polymerized, and the remainder of the polymerization batch issubsequently fed to the polymerization zone continuously, in steps orwith superposition of a concentration gradient, usually via a pluralityof spatially separate feed streams, of which one or more contain themonomers in pure or emulsified form, while maintaining thepolymerization. In an applicationally advantageous manner, the initiallyintroduced mixture and/or the monomer feed stream contains small amountsof emulsifiers, generally less than 0.5% by weight, based on the totalamount of monomers to be polymerized, in order to reduce the surfacetension of the dispersion medium and thus to simplify stirring in. Themonomers are therefore frequently fed to the polymerization zone afterpre-emulsification with these assistant emulsifiers. Due to the highwater solubility of the sugared starches to be used according to theinvention, the feed process can be designed in a particularly simplemanner by initially introducing all of the sugared starch to be used indissolved form in an aqueous mixture; pregelling is unnecessary. Thismeans that the aqueous solution produced on partial hydrolysis of thestarting starch can, after the hydrolysis has been terminated, forexample by neutralization of the catalytic acid and cooling, be furtherused directly for the aqueous emulsion polymerization. Prior isolation,for example by spray drying, of the sugared starch is unnecessary.

Suitable free-radical polymerization initiators are all those which arecapable of initiating a free-radical aqueous emulsion polymerization.These may be either peroxides, for example alkali metal peroxydisulfatesor H₂ O₂, or azo compounds.

Also suitable are combined systems comprising at least one organicreducing agent and at least one peroxide and/or hydroperoxide, e.g.tert-butyl hydroperoxide and the sodium metal salt ofhydroxymethanesulfinic acid or hydrogen peroxide and ascorbic acid. Alsosuitable are combined systems additionally containing a small amount ofa metal compound which is soluble in the polymerization medium and whosemetallic component can exist in more than one oxidation state, e.g.ascorbic acid/iron(II) sulfate/hydrogen peroxide, where ascorbic acid isalso frequently replaced by the sodium metal salt ofhydroxymethanesulfinic acid, sodium sulfite, sodium hydrogen sulfite orsodium metal bisulfite and hydrogen peroxide is frequently replaced bytert-butyl hydroperoxide or alkali metal peroxydisulfates and/orammonium peroxydisulfates. In the combined systems, it is furthermoreexpedient to use the sugared starches as the reducing component. Ingeneral, the amount of free-radical initiator systems employed is from0.1 to 2% by weight, based on the total amount of the monomers to bepolymerized. Particularly preferred initiators are ammonium and/oralkali metal peroxydisulfates, alone or as a constituent of combinedsystems. Particular preference is given to sodium peroxydisulfates.

The manner in which the free-radical initiator system is added to thepolymerization reactor during the free-radical aqueous emulsionpolymerization according to the invention is of somewhat secondaryimportance. It can either all be introduced into the polymerizationreactor at the beginning, or added continuously or stepwise as it isconsumed during the free-radical aqueous emulsion polymerization. Indetail, this depends in a manner known to an average person skilled inthe art both from the chemical nature of the initiator system and on thepolymerization temperature. In a preferred procedure, some is introducedat the beginning and the remainder is added to the polymerization zoneas it is consumed.

It is of course also possible to carry out the free-radical aqueousemulsion polymerization according to the invention undersuperatmospheric or reduced pressure.

The aqueous polymer dispersions according to the invention are generallyprepared with total solids contents of from 15 to 65% by weight,particular preference being given for applicational reasons to thosewhich contain from 10 to 75% by weight, very particularly preferablyfrom 20 to 60% by weight, based on the polymerized monomers, of addedsugared starches according to the invention.

The aqueous polymer dispersions according to the invention are extremelyversatile. They are particularly suitable as adhesives, as binders forcarpet backings, as binders for paper coatings, as additives in mineral,e.g. hydraulically setting, binders, as fiber sizes, as binders forfinely divided mineral and/or organic materials for the production ofmoldings (e.g. chipboard), in particular for metal casting, or paintsand plasters, as thickeners and as binders for the production ofabrasives based on finely divided abrasive particles bonded to oneanother and/or to a support. Particularly advantageous here is theincreased film-formation capacity of the aqueous polymer dispersionsaccording to the invention and the increased tear strength (inparticular at elevated temperature) of the resultant films. Theseproperties mean that the aqueous polymer dispersions according to theinvention are also suitable for the production of coatings, inparticular for covering and or embedding pharmaceutical activeingredients to be administered orally.

A further notable property of the aqueous polymer dispersions accordingto the invention is that they can generally be converted, in a mannerknown per se, to redispersible polymer powders (for example by spraydrying or roll drying or suction-filter drying). In general, it is notnecessary in this respect, even in the case of soft polymers (glasstransition temperature below 30° C.), to use known aids, such asspraying aids (for example polyvinyl alcohols or finely dividedsilicates), in particular if the content of sugared starches to be usedaccording to the invention in the aqueous polymer dispersions accordingto the invention is greater than 10% by weight, based on the amount ofpolymerized monomers.

It is of course possible to use known drying aids. The redispersibilityof the powders is generally retained even on extended storage. It is ofparticular interest that the powders are generally still redispersibleeven if no monomers containing ionic groups have been copolymerized. Itshould again be particularly emphasized that the aqueous polymerdispersions according to the invention have increased flowability, evenat high solids contents, and a long shelf life.

Aqueous polymer dispersions according to the invention which can be usedin an advantageous manner with particular applicational versatility arethose whose polymers are obtainable by free-radical polymerization ofmonomer mixtures which have the following monomer composition,comprising

from 39 to 69% by weight of at least one ester of α,β-monoethylenicallyunsaturated mono- and dicarboxylic acid having 3 to 6 carbon atoms withalkanols having 1 to 6 carbon atoms (monomer a),

from 30 to 60% by weight of styrene (monomer b),

from 1 to 10% by weight of at least one monomer from the groupcomprising α,β-monoethylenically unsaturated carboxylic acids having 3to 6 carbon atoms, and amides and nitriles thereof, (monomer c) and

from 0 to 10% by weight of one or more monomers from the groupcomprising N-alkylolamides of α,β-monoethylenically unsaturatedcarboxylic acids having 3 to 6 carbon atoms, with 1 to 4 carbon atoms inthe alkyl group, and monomers having up to 25 carbon atoms andcontaining two non-conjugated, ethylenically unsaturated double bonds(monomer d),

and which contain, based on the polymerized monomers, from 1 to 120% byweight, preferably from 10 to 65% by weight, particularly preferably 35to 55% by weight, of at least one added sugared starch to be usedaccording to the invention. Their total solids content is preferablyfrom 40 to 60% by weight.

It is of course possible to add assistants known per se for theparticular application, for example film-forming assistants or fillers,to the aqueous polymer dispersions according to the invention.Correspondingly, the internal strength of the abovementioned preferredaqueous polymer dispersions can be varied by replacing some or all ofthe monomers d) by other crosslinking monomers mentioned in thispublication.

These aqueous polymer dispersions according to the invention, obtainableby free-radical aqueous emulsion polymerization of monomer mixturescomprising monomers a, b, c and, if desired, d are particularly suitableas binders for foundry sands for the production of cores and molds formetal casting, for the production of ingot mold insulating board basedon finely divided paper and, if desired, finely divided mineralmaterials, and for the production of abrasives based on finely dividedabrasive particles bonded to one another and/or to a support by means ofa binder. Suitable foundry sand generally comprises granular quartzsand, but in certain cases also chromite, zirconium or olivine sand.Fireclay, magnesite, sillimanite or corundum materials are also used.The mean particle diameter (maximum diameter) is normally from 0.05 to0.6 mm. Conversion of the foundry sands to foundry moldings is generallycarried out by mixing the sands with the aqueous polymer dispersionsaccording to the invention while establishing the desired bindercontent, in general (calculated in dry form) from 0.1 to 10% by weight,preferably from 0.1 to 5% by weight, based on the amount of foundrysand, transferring the mixture (referred to here as mortar) into a mold(negative), if desired compacting the material by exerting pressure, andsubsequently curing the material. It is noteworthy that, on use of theaqueous polymer dispersions according to the invention, which areusually used for this purpose with a total solids content of from 40 to60% by weight, the curing process does not necessarily require the useof elevated temperatures (normally from 50° to 250° C.), but thoroughcuring also takes place at a satisfactory rate when left to itself atroom temperature. The curing can also be achieved in an applicationallyparticularly elegant manner by exposing the material to be cured to theaction of microwaves. In this respect, aqueous polymer dispersionsaccording to the invention which are obtainable by free-radical aqueousemulsion polymerization of mixtures of monomers a, b, c and d whosemonomer composition is selected so that a polymer built up only frommonomers a, b and c would have a glass transition temperature in therange from 0° to 40° C., are recommended with particular advantage.

According to Fox (T. G. Fox, Bull. Am. Phys. Soc. (Ser. II) 1 (1956)123), the glass transition temperature of copolymers is given, to aclose approximation, by: ##EQU1## where X¹, X², . . . , X^(s) denote theproportions by weight of monomers 1, 2, . . . , s, and Tg¹, Tg², . . . ,Tg^(s) denotes the glass transition temperatures of the respectivepolymers built up only from one of the monomers 1, 2, . . . , s, inKelvin. The glass transition temperatures of the monomers a, b and c areessentially known and are listed, for example, in J. Brandrup, E. H.Immergut, Polymer Handbook, 1st Edn., J. Wiley, New York 1966, and 2ndEdn., J. Wiley, New York 1975.

A further advantage of the use of the aqueous polymer dispersionaccording to the invention is that the cured molding has increasedflexural strength, in particular at elevated temperature. This ensuresincreased dimensional stability in the presence of hot molten metal.Furthermore, the negative mold is simple to clean with water afterremoval of the molding, and the molding itself can, if required, beconverted back into uncured mortar by adding water (redispersibility),which is equivalent to an infinitely long processability of the mortar.If the polymers also contain incorporated monomers d, particularly highflexural strengths result. In addition, the flexural strengths can befurther increased by adding up to 20% by weight, based on the solidscontent of the aqueous polymer dispersions determined without thesugared starch, of saturated dialdehydes, preferably those of theformula I ##STR1## before the aqueous polymer dispersions to be usedaccording to the invention are used, dialdehydes of the formula I wheren=0 to 2 being preferred. Other suitable additives for increasing theflexural strengths are condensation products based on formaldehyde,melamin, phenol and/or urea, e.g. Urecoll® 118. The amount to be usedcan be up to 250% by weight, based on the solids content of the aqueouspolymer dispersions according to the invention determined without thesugared starch.

However, the last-mentioned additives only have an advantageous effectin the manner described if the curing is carried out at elevatedtemperature, in general from 100° to 250° C., or in the presence ofacid. The latter can be achieved in a simple manner by adjusting the pHof the dispersion medium of the aqueous polymer dispersions to be usedaccording to the invention to from 1 to 5, preferably to from 2 to 3. Ifflexural strengths within usual limits are required, it is preferred touse no additives. The moldings obtainable have, as further properties:

good resistance to erosion by molten metal

smooth and closed surfaces of the casting

good release after casting

minimal subsequent cleaning for the castings

minimal evolution of toxic gases during casting.

The polymer dispersions according to the invention, which areparticularly suitable for the production of foundry moldings, are, inthe same way, particularly suitable for the production of abrasivesbased on finely divided abrasive particles which are bonded to oneanother and/or to a support by means of a binder. Particularly suitablefinely divided abrasive particles are fused or sintered corundum,zirconium corundum, silicon carbide and emery. Suitable supportmaterials include flexible substrates, e.g. paper, vulcanized fiber,woven fabrics, knitted fabrics, nonwovens based on natural and/orsynthetic fibers, plastic films or metal foils. In general, abrasives ofthis type are produced by first applying to the support a so-called makecoat, into which, in the wet state, the abrasive particles are embedded.After a first fixing of the abrasive particles by drying (curing), asecond, so-called size coat is generally applied in order to improve theembedding and reinforce the particles. In principle, the make coat andsize coat can comprise different binders. According to the invention, atleast one of the two, preferably the size coat and particularlypreferably both, comprise the aqueous polymer dispersions according tothe invention. Typical requirements made of binders suitable for theproduction of abrasives are:

good adhesion, both to the substrate and to the abrasive particles,

curable rapidly under gentle conditions,

very low loading of the support material,

high heat distortion resistance,

increased flowability on application and

good mechanical properties during grinding (formation of hard, toughfilms).

These requirements are achieved entirely satisfactorily by the aqueouspolymer dispersions according to the invention. Thus, curing when theyare used does not necessarily require elevated temperatures, but can becarried out at room temperature and particularly advantageously underthe action of microwaves. This is particularly gentle for the supportmaterial and avoids extreme removal of water, making complexregeneration of the carrier material in climatic zones unnecessary.

Their favorable flow behavior proves to be particularly advantageous ifthe aqueous polymer dispersions according to the invention are used as asize coat, since it enables the binder to penetrate into the intersticesbetween the abrasive particles.

In addition, the binders according to the invention are distinguished,in particular, by increased heat distortion resistance, so that theabrasive particles remain fixed in their position even at the elevatedtemperatures (150° C. or more) which occur during grinding. Loosening ofthe abrasive particles (which reduces the abrasive effect) or evendislodgement is thus suppressed. The make coat is usually applied in adry film thickness of from 10 to 100 μm and the size coat in a dry filmthickness of from 20 to 103 μm.

When employed as binders for paper coating compositions, the aqueouspolymer dispersions according to the invention give paper increased wetand dry pick resistance.

In the examples below, the sugared starches used were the C* PURproducts 01906, 01908, 01910, 01915, 01921, 01924, 01932 and 01934 fromCerestar Deutschland GmbH, D-1150 Krefeld 12. Essentially all have abimodal molecular weight distribution and are characterized as follows:

    ______________________________________                                                                  % by wt.      η.sup.40                          Type  M.sub.w     U       <1000   DE    [Pa.s]                                ______________________________________                                        01906 20080       10.9    12.2    2-5   --                                    01908 19290       10.0    15.9     8-10 0.056                                 01910 10540-12640 8.5-9.9 24.7-26.4                                                                             11-14 0.030                                 01915 6680-8350   6.8-8.4 32.9-34.7                                                                             17-19 0.021                                 01921 6700        7.4     39.1    20-23 0.017                                 01924 4730        6.8     53.6    26-30 0.014                                 01932 4500        7.9     63.2    33-35 0.011                                 01934 3000        6.0     68.4    36-39 0.009                                 ______________________________________                                    

Determination of M_(n) by means of vapor pressure osmometry gave thefollowing values for the preferred types 01910 and 01915:

1560 g/mol (1910)

980 g/mol (1915)

EXAMPLES Example 1

Aqueous polymer dispersions DX according to the invention (the data in %by weight are always based on the amount of polymerized monomers, unlessstated otherwise)

D1 to D10: General preparation procedure

A mixture comprising

513 g of water

120 g of sugared starch (20% by weight)

60 g of feed 1 and

39 g of feed 2

was heated at 85° C. and kept at this temperature for 15 minutes. Theremainder of feeds 1 and 2 were subsequently fed continuously to thepolymerization zone beginning at the same time (feed 1 over the courseof 2.5 hours, feed 2 over the course of 3 hours) while maintaining thetemperature of 85° C. The mixture was subsequently polymerized for afurther hour (85° C.) and then cooled to room temperature.

Feed 1:

330 g of n-butyl acrylate (55% by weight)

258 g of styrene (43% by weight)

12 g of acrylic acid (2% by weight)

Feed 2:

4.8 g of sodium peroxydisulfate (0.8% by weight) in 190 g of water

The same free-radical aqueous emulsion polymerization was repeated, butthe initial mixture contained 300 g of sugared starch dissolved in 710 gof water. Aqueous polymer dispersions were obtained whose total solidscontent was essentially 50% by weight and which essentially contained noflocculations. The Brookfield viscosity [BV(mPa.s)] of these polymerdispersions was determined at 25° C. by means of a Brookfield RVTviscometer every 3 months, in each case at 20 and 100 revolutions perminute, and the light transparency (LT) was also determined. The datafor the light transparency are based on a 0.1% strength by weightaqueous polymer dispersion and a cell thickness of 25 mm. Themeasurement was carried out using a commercially available photometerrelative to water, which was randomly assigned an LT of 100. The LT is ameasure of the mean particle size. The results obtained are shown inTable 1.

                  TABLE 1                                                         ______________________________________                                        Type and                                                                      % by wt.    BV(20)  BV (100)                                                  of sugared  in each      BV(20)  BV (100)                                     starch      case immediately                                                                           after 3 months                                                                             LT                                      ______________________________________                                        D1   01915/20   140     106    130   100    45                                D2   01921/20   52      67     50    63     32                                D3   01924/20   40      57     40    56     29                                D4   01932/20   32      50     32    50     24                                D5   01934/20   29      48     30    46     20                                D6   01915/50   192     151    170   140    71                                D7   01921/50   54      83     60    80     58                                D8   01924/50   48      74     42    72     55                                D9   01932/50   32      54     30    52     42                                D10  01934/50   27      48     25    47     37                                ______________________________________                                    

D11 to D13:

D11: A mixture comprising

200 g of water

25 g of sugared starch 01910 (5% by weight)

71 g of feed 1 and

10 g of feed 2

was heated to 85° C. and kept at this temperature for 15 minutes. Theremainder of feeds 1 and 2 were subsequently fed continuously to thepolymerization zone commencing at the same time (feed 1 over the courseof 2.5 hours, feed 2 over the course of 3 hours), while maintaining thetemperature of 85° C. The mixture was then polymerized for a furtherhour (85° C.) and cooled to room temperature. The total solids contentof the resultant aqueous polymer dispersion, which was essentially freefrom flocculation, was essentially 50% by weight.

    ______________________________________                                        Feed 1:                                                                              450 g of n-butyl acrylate (90% by weight)                                     40 g of styrene (8% by weight)                                                10 g of acrylic acid (2% by weight)                                           pre-emulsified in 204 g of water by means of                                  1.5 g (0.3% by weight) of sodium salt of dodecyl-                             benzenesulfonic acid.                                                  ______________________________________                                    

Feed 2:

2.5 g of sodium peroxydisulfate dissolved in 100 g of water (0.5% byweight).

D12 and D13:

At the total solids content corresponding to D11, the monomer mixtures

D12:

55% by weight of n-butyl acrylate

43% by weight of styrene

2% by weight of acrylic acid and

D13:

50% by weight of n-butyl acrylate

45% by weight of styrene

5% by weight of acrylic acid

were polymerized in a corresponding manner. However, the proportion ofsugared starch 01910 was 20% by weight (D12) or 40% by weight (D13), andthe sodium salt of dodecylbenzenesulfonic acid was in both cases used ina proportion of 0.1% by weight.

The BV and LT valves of the resultant aqueous polymer dispersions, whichin all cases contained essentially no flocculations, are shown in Table2.

                  TABLE 2                                                         ______________________________________                                        BV(20)     BV(100)                                                            in each case    Months                                                        immediately     thereafter                                                                              BV(20)   BV(100)                                    ______________________________________                                        D11   63       69       8       50     63                                     D12   88       91       10      75     88                                     D13   132      122      3       124    117                                    ______________________________________                                    

D14:

A mixture comprising

394 g of water

300 g of sugared starch 01915 (100% by weight)

40 g of feed 1 and

40 g of feed 2

was heated at 85° C. and kept at this temperature for 5 minutes. Theremainder of feeds 1 and 2 was subsequently fed continuously to thepolymerization zone commencing at the same time (feed 1 over the courseof 2.5 hours, feed 2 over the course of 3 hours), while maintaining thetemperature of 85° C. The mixture was subsequently polymerized for afurther hour (85° C.) and cooled to room temperature. The solids contentof the resultant dispersion was 50.6% by weight; there were essentiallyno flocculations.

    ______________________________________                                        Feed 1:  294 g of styrene (98% by weight)                                              6 g of methacrylic acid (2% by weight)                                        pre-emulsified in 101 g of water by means of                                  2 g of a 15% strength by weight aqueous solu-                                 tion of the sodium salt of dodecylbenzenesul-                                 fonic acid (0.1% by weight).                                         ______________________________________                                    

Feed 2:

2.4 g of sodium peroxydisulfate dissolved in 100 g of water (0.8% byweight).

Determination of the Brookfield viscosities and LT gave the followingresults:

    ______________________________________                                        BV(20)    BV(100)  BV(20)     BV(100)                                         immediately     after 3 months LT                                             ______________________________________                                        76        98       70         112    39                                       ______________________________________                                    

D15:

A mixture comprising

549 g of water

199.6 g of sugared starch 01910 (100% by weight)

24 g of feed 1 and

30 g of feed 2

was treated at 85° C. and kept at this temperature for 15 minutes. Theremainder of feeds 1 and 2 was subsequently fed continuously to thepolymerization zone commencing at the same time (feed 1 over the courseof 2 hours, feed 2 over the course of 2.5 hours), while maintaining thetemperature of 85° C. The mixture was subsequently polymerized for afurther hour (85° C.) and cooled to room temperature. The solids contentof the aqueous polymer dispersions, which was essentially free fromflocculations, was 29.4% by weight.

    ______________________________________                                        Feed 1:  196 g of styrene (98% by weight)                                              4 g of methacrylic acid (2% by weight)                                        pre-emulsified in 271 g of water by means of                                  6.7 g of a 15% strength by weight aqueous                                     solution of the sodium salt of dodecylbenzene-                                sulfonic acid (0.5% by weight).                                      ______________________________________                                    

Feed 2:

1.6 g of sodium peroxydisulfate in 100 g of water (0.8% by weight ).

The BV values were determined as follows:

    ______________________________________                                        BV(20)   BV(100)       BV(20)  BV(100)                                        immediately     after 3 months                                                ______________________________________                                        17       32            17      30                                             ______________________________________                                    

D16 to D18:

D16:

A mixture comprising

203 g of water

6 g of sugared starch 01910 (1% by weight)

81 g of feed 1 and

39 g of feed 2

was heated to 90° C. and kept at this temperature for 30 minutes. Theremainder of feeds 1 and 2 was subsequently fed continuously to thepolymerization zone commencing at the same time (feed 1 over the courseof 2.5 hours, feed 2 over the course of 3 hours), while maintaining thetemperature of 90° C. The mixture was subsequently polymerized for afurther hour (90° C.) and cooled to room temperature.

    ______________________________________                                        Feed 1:  330 g of n-butyl acrylate (55% by weight)                                     258 g of styrene (43% by weight)                                              12 g of acrylic acid (2% by weight)                                           pre-emulsified in 202 g of water by means of 4 g                              of a 15% strength by weight aqueous solution of                               the sodium salt of dodecylbenzenesulfonic acid                                (0.1% by weight).                                                    ______________________________________                                    

Feed 2:

4.8 g of sodium peroxydisulfate in 190 g of water (0.8% by weight).

D17 and D18:

As for D16, but, instead of 203 g of water/6 g of 01910, the initialmixture contained 214 g of water/18 g of 1910 (3% by weight, D17) or 225g of water/30 g of 01910 (5% by weight, D18).

The BV and LT values of the resulting aqueous polymer dispersions, whichin all cases contain essentially no flocculations, are shown in Table 3.The solids content was essentially 50% by weight in all cases.

                  TABLE 3                                                         ______________________________________                                        BV(20)    BV(100)  B(20)   BV(100)                                            immediately    after 11 months                                                                            LT      - d [nm]                                  ______________________________________                                        D16   30      49       25    42     13    407                                 D17   40      59       33    50     25    310                                 D18   92      92       62    76     51    209                                 ______________________________________                                    

d is the mean particle diameter and was determined by means of a photoncorrelation spectrometer (Malvem Autosizer 2C).

D19:

A mixture comprising

549 g of water

120 g of sugared starch 01910 (20% by weight)

2 g of a 15% strength by weight aqueous solution of the sodium salt oflaurylsulfonic acid (0.05% by weight)

60 g of feed 1 and

31 g of feed 2

was heated to 95° C. and kept at this temperature for 15 minutes. Theremainder of feeds 1 and 2 was subsequently fed continuously to thepolymerization zone commencing at the same time (feed 1 over the courseof 2.5 hours, feed 2 over the course of 3 hours), while maintaining thetemperature of 95° C. The mixture was subsequently polymerized for afurther hour (85° C.) and cooled to room temperature. An aqueous polymerdispersion was obtained which contained essentially no flocculations andwhose solids content was 50% by weight.

    ______________________________________                                        Feed 1:  480 g of ethyl acrylate (68% by weight)                                       192 g of methyl methacrylate (32% by weight)                         ______________________________________                                    

Feed 2:

1.8 g of sodium peroxydisulfate in 100 g of water (0.3% by weight )

    ______________________________________                                        BV(20)    BV(100)  BV(20)     BV(100)                                         immediately     after 4 months LT                                             ______________________________________                                        64        86       53         78     58                                       ______________________________________                                    

D20 [D21]:

A mixture comprising

206 g [227 g] of water

50 g [100 g] of sugared starch 01910 (10% by weight [20% by weight ])

16.7 g of a 30% strength by weight aqueous hydrogen peroxide solution

[16.7 g of a 15% strength by weight aqueous solution of the sodium saltof laurylsulfonic acid]

47 g of feed 1

was warmed to 50° C., and 15 g of feed 2 were added. The reactionmixture was subsequently warmed to 70° C. and kept at this temperaturefor 15 minutes. The remainder of feeds 1 and 2 was subsequently fedcontinuously to the polymerization zone commencing at the same time(feed 1 over the course of 2.5 hours, feed 2 over the course of 3hours), while maintaining the temperature of 70° C. The mixture wassubsequently polymerized for a further 30 minutes (70° C.) and cooled toroom temperature.

    ______________________________________                                        Feed 1:    270 g of ethyl acrylate (54% by weight)                                       230 g of methacrylic acid (46% by weight)                                     33.3 [16.7 g] of a 15% strength by weight                                     aqueous solution of the sodium salt lauryl-                                   sulfonic acid (0.1% by weight [0.05% by                                       weight])                                                                      413 g of water                                                     ______________________________________                                    

Feed 2:

3 g of ascorbic acid and 0.05 g of Mohr's salt (=(NH₄)₂ Fe(SO₄)₂ 6 H₂ O)in 150 g of water.

About 40% strength by weight aqueous polymer dispersions were obtainedwhich were essentially free from flocculations. The BV and LT valueswere determined as follows:

    ______________________________________                                               BV(20)      BV(100)                                                              after 4 months                                                                              LT                                                    ______________________________________                                        D20      18            33       33                                            D21      27            48       61                                            ______________________________________                                    

D22:

A mixture of

1,443.2 g of water

2,500 g of a 20% strength by weight aqueous solution of sugared starch01915 (25% by weight)

0.06 g of FeSO₄.7 H₂ O

3.33 g of a 60% strength by weight aqueous solution of the sodium saltof di-2-ethylhexyl sulfosuccinate (0.1% by weight)

328.6 ml of feed 1 and

80 ml of feed 2

was heated to 85° C., and the reaction mixture was kept at thistemperature for 15 minutes. The remainder of feeds 1 and 2 wassubsequently fed continuously to the reaction zone commencing at thesame time (feed 1 over the course of 3 hours, feed 2 over the course of4 hours), while maintaining the temperature of 85° C. The mixture wassubsequently polymerized for a further 3 hours (85°) and cooled to roomtemperature. An aqueous polymer dispersion was obtained which wasessentially free from flocculations and had a total solids content of41% by weight.

    ______________________________________                                        Feed 1:  2000 g of butadiene (3.23 l) (100% by weight)                                 60 g of tert-dodecyl mercaptan (3% by weight)                        ______________________________________                                    

Feed 2:

24 g of sodium peroxydisulfate in 376 g of water (1.2% by weight)

The BV and LT values were determined as follows:

    ______________________________________                                        BV(20)        BV(100)                                                         after 3 months      LT                                                        ______________________________________                                        22             42       59                                                    ______________________________________                                    

D23:

A mixture of

247 g of water

90 g of sugared starch 01910 (20% by weight)

63 g of feed 1 and

37 g of feed 2

was heated to 80° C. and kept at this temperature for 15 minutes. Theremainder of feeds 1 and 2 was subsequently fed continuously to thereaction zone commencing at the same time (feed 1 over the course of 2.5hours, feed 2 over the course of 3 hours), while maintaining thetemperature of 80° C. The mixture was subsequently polymerized for afurther hour (80° C.) and cooled to room temperature. An aqueous polymerdispersion was obtained which was essentially free from flocculationsand had a total solids content of 50.2% by weight.

    ______________________________________                                        Feed 1:   247.5 g of n-butyl acrylate (55% by weight)                                   193.5 g of styrene (43% by weight)                                            9 g of acrylic acid (2% by weight)                                            pre-emulsified in 173 g of water by means of                                  4.5 g of a 20% strength by weight aqueous                                     solution of a mixture of ethoxylated fatty                                    alcohols (degree of ethoxylation 18, C.sub.16/18)                             (0.2% by weight)                                                    ______________________________________                                    

Feed 2:

2.25 g of sodium peroxydisulfate (0.5% by weight) in 90 g of water

The BV values were determined as follows:

    ______________________________________                                        BV(20)  BV(100)        BV(20)  BV(100)                                        immediately     after 1 year                                                  ______________________________________                                        188     121            140     106                                            ______________________________________                                    

Example 2

Applications of aqueous polymer dispersions according to the invention

A1: Binder for a paper coating composition

A 60% strength by weight paper coating composition which, apart fromwater, had the following composition:

80 g of china clay 5 PS (kaolin)

20 g of Omyalite 90 (chalk)

0.2 g of Polysalz®S (low-molecular-weight polyacrylic acid, dispersantfor kaolin and chalk)

11.5 g of aqueous polymer dispersion D23 according to the invention fromExample 1 (calculated in dry form)

0.3 g of Sterocoll®D (calculated in dry form) (aqueous dispersion of anacrylate polymer with a high acrylic acid content, thickener) and

0.6 g of Blankophor®PSG-fl. (optical brightener)

was coated onto untreated paper (supplied by Scheufelen, Oberlennigen,Germany) having a basis weight of 70 g/m². The application rate was 12g/m² (dry). The coating was subsequently dried at 120° C. for 5 minutesand then conditioned at 23° C. and 50% relative atmospheric humidity for8 hours. The coated paper was then glazed in a roll press under a linearpressure of 125 kp/cm, and briefly conditioned again, and the coatingproperties were then determined. To this end, the dry pick resistanceDPR (data for the critical pick rate; high rates correspond to high drypick resistances) and the wet pick resistance WPR (data for theresultant color density in %; high values correspond to high wet pickresistances) were determined using a commercially available IGTapparatus. Furthermore, the print gloss PG was determined using aLehmann tester at an angle of 45°. For comparison, the procedure wasrepeated using a paper coating composition in which the 11.5 g of D23(calculated in dry form) was replaced by 11.5 g of a polymer dispersion(Acronal®S 320D, calculated in dry form) commercially available for thispurpose and stabilized by means of an emulsifier. The results obtainedare shown in Table 4.

                  TABLE 4                                                         ______________________________________                                                  TR (cm/s) NR (%)   DG (%)                                           ______________________________________                                        D23         64          10.6     8.5                                          Acronal S 320D                                                                            46          30.6     28                                           ______________________________________                                    

A2: Spray drying for the preparation of powders

Aqueous polymer dispersion D13 from Example 1 was spray-dried by meansof a spray drier (Niro Minor) without the addition of furtherassistants.

Inlet temperature: 130° C.

Outlet temperature: 80° C.

A white polymer powder which was redispersible in an entirelysatisfactory manner was obtained.

A3: Aluminum paper lamination

70 g of water and, as thickener, 3 g of Collacral®HP (30% strength byweight aqueous solution of a copolymer based on acrylic acid andacrylamide emulsified in an aliphatic crude oil fraction) were stirredinto 50 g of aqueous polymer dispersion D13 from Example 1 after removalof the residual monomers. 2 g/m² (dry) of this lamination adhesive K1were applied to the matt side of a 0.0095 mm thick aluminum foil matt onone side. Buxine paper (80 g/m²) was rolled onto the wet adhesive bed,and the resultant substrate was dried at 50° C. for 3 minutes. The paperwas subsequently cut to a format of 20 cm×20 cm, and this section wassubjected to a pressure of 100 bar at 90° C. for 3 minutes. The laminatewas subsequently subjected to the following tests:

1) Adhesion test

The adhesion of the Buxine paper after storage for 30 minutes in coldwater (20° C.) and after storage for 5 minutes in hot water (100° C.)was tested by hand. The assessment was carried out by scores 1 to 5.

1=paper tears over the entire area

2=paper tears over part of the area

3=good adhesion with adhesive failure in the foil or paper

4=weak adhesion with adhesive failure in the foil or paper

5=no adhesion.

Heat-sealing resistance:

The sections were subjected with the aluminum side up to a temperaturerising from 180° C. to 250° C. in a heat-sealing unit at 0.25 secondintervals under a pressure of 2.5 bar, the temperature rising in 10° C.steps. The test was terminated when bubble formation was clearlyvisible. The heat-sealing resistance is important for sealing on plasticfilms.

Odor test:

Sections measuring 9 cm×18 cm were subjected to a temperature of 110° C.for 15 seconds and then sealed in 370 ml bottles and stored at 50° C.for 2 hours. The bottles were subsequently opened and tested for odor.

1=no odor

2=weak odor

3=odor

4=strong odor

The experiment series was repeated with a lamination adhesive K2 which,in contrast to K1, additionally contained 3.1 g of a 40% strength byweight aqueous solution of glyoxal. The results are shown in Table 5.

                  TABLE 5                                                         ______________________________________                                                      K1        K2                                                    ______________________________________                                        Adhesion cold   1           1                                                 Adhesion hot    1           1                                                 Heat-sealing resistance                                                                       up to 210° C.                                                                      up to 240° C.                              Odor            2           2                                                 ______________________________________                                    

A4: Abrasive articles based on finely divided abrasive particles bondedto a support by means of an aqueous polymer dispersion according to theinvention

100 g of various aqueous polymer dispersions according to the inventionfrom Example 1, to some of which 0.062 part by weight of glyoxal hadbeen added per part by weight of sugared starch present, were mixed with1 g of Lumiten® (wetting agent) and applied to a support paper at anapplication rate of 20 g/m² (dry). Semiprecious corundum 60 wasscattered into the wet coating, and the paper coated in this way wasdried at 90° C. for 3 minutes. The same binder was subsequently appliedas a size coat at an application rate of 60 g/m² (dry) and likewisedried (30 minutes). The resultant abrasive paper was tested by means ofan APG 100/20 abrasion tester (Maag & Schank, Gomaringen). The testspecimens used were specimens measuring 40 mm×20 mm×5 mm made from rigidPVC. In the test, an area of 20 mm×5 mm was rubbed off by 500 strokes(load 1 kg), with the abrasive paper being moved to and fro beneath thetest specimen for a length of 10.5 cm. A measure of the quality of thebinder is the abrasion, which is defined as follows: ##EQU2##

The results are shown in Table 6.

                  TABLE 6                                                         ______________________________________                                                           Drying                                                                        temperature                                                Binder  Glyoxal    (°C.) top layer                                                                    Abrasion (%)                                   ______________________________________                                        D13     -          90          8.7                                            D13     +          90          10.7                                           D24*    +          150         7                                              D25*    +          150         7.5                                            ______________________________________                                         *D24 and D25 prepared as for D13, but with a different monomer                composition:                                                             

D24:

50% by weight of n-butyl acrylate,

45% by weight of styrene

5% by weight of methacrylic acid

D25:

50% by weight of n-butyl acrylate,

40% by weight of styrene

10% by weight of acrylic acid.

A5: Foundry moldings containing binders according to the invention

1000 g of quartz sand H 33 were mixed with various amounts (indicated inTable 7 in % by weight based on sand, calculated in dry form) of aqueouspolymer dispersions according to the invention having a total solidscontent of 50% by weight, to some of which 0.125 part by weight ofglyoxal or 1.075 parts by weight of Urecoll 118 had been added per partby weight of sugared starch present. The mixture was subsequentlyintroduced into a mold and compacted three times by means of a ram (G.Fischer A G, Schaffhausen, Switzerland) having a weight of 6.7 kg from afull height of 5 cm. The resultant moldings (Fischer bars) had thefollowing dimensions, with rounded ends:

length about 173 mm

width about 22 mm

height about 22 mm

The bars were then dried under various conditions, and the core flexuralstrength was subsequently determined at room temperature using a testerfrom G. Fischer A G (Schaffhausen, Switzerland).

The preparation of the novel aqueous polymer dispersions usedcorresponded to D12 from Example 1, but in some cases with a differentmonomer composition and, based on the monomers, different amounts ofsugared starch. Details and results are shown in Table 7.

The following abbreviations were used for the monomers:

BA=n-butyl acrylate

St=styrene

AA=acrylic acid

MAmol=n-methylolmethacrylamide

BDA=butanediol diacrylate

DVB=divinylbenzene

                                      TABLE 7                                     __________________________________________________________________________                   % by                                                                          weight                          Core flexural                  Monomer composition                                                                          of   Amount of       Drying                                                                             Tempera-                                                                            strength                       (% by weight)  starch                                                                             binder                                                                              Glyoxal                                                                            Urecoll                                                                            time ture (°C.)                                                                   (N/mm.sup.2)                   __________________________________________________________________________    55 BA 43 St 2 AA                                                                             40   1     -    -    10                                                                              min                                                                              150   1.8                            55 BA 43 St 2 AA                                                                             40   2     -    -    10                                                                              min                                                                              150   4.9                            55 BA 43 St 2 AA                                                                             40   3     -    -    10                                                                              min                                                                              150   6.0                            55 BA 43 St 2 AA                                                                             40   3     -    -    30                                                                              min                                                                              150   5.8                            55 BA 43 St 2 AA                                                                             40   3     +    -    30                                                                              min                                                                              150   6.2                            55 BA 43 St 2 AA                                                                             50   2     -    -    10                                                                              min                                                                              150   4.9                            45 BA 53 St 2 AA                                                                             40   2     -    -    10                                                                              min                                                                              150   6.2                            65 BA 33 St 2 AA                                                                             40   2     -    -    10                                                                              min                                                                              150   3.8                            52 BA 41 St 2 AA 5 MAmol                                                                     50   2     -    -    10                                                                              min                                                                              150   4.5                            55 BA 42.5 St 2 AA 0.5 BDA                                                                   50   2     -    -    10                                                                              min                                                                              150   5.2                            54 BA 42 St 2 AA 2 DVB                                                                       50   2     -    -    10                                                                              min                                                                              150   5.0                            50 BA 45 St 5 AA                                                                             40   2     -    -    10                                                                              min                                                                              150   5.2                            50 BA 45 St 5 AA                                                                             40   2     -    +    10                                                                              min                                                                              150   7.9                            45 BA 53 St 2 AA                                                                             40   2     -    +    10                                                                              min                                                                              150   8.7                            45 BA 53 St 2 AA                                                                             40   2     -    +    10                                                                              min                                                                              150   2,6 (at                                                                       200° C.)                55 BA 42.5 St 2 AA 0.5 DVB                                                                   50   2     -    -    3 min                                                                              Microwave                                                                           5.0                                                                     1300W                                50 BA 45 St 5 AA                                                                             40   2     -    -    4 h   25   2.5                            50 BA 45 St 5 AA                                                                             40   2     -    -    6 h   25   5.1                            __________________________________________________________________________

We claim:
 1. An aqueous polymer dispersion obtained by free-radicalpolymerization of a monomer mixture containing from 39 to 69% by weightof at leastone ester of α,β-monoethylenically unsaturated mono- anddicarboxylic acids having 3 to 6 carbon atoms with alkanols having 1 to6 carbon atoms, from 30 to 60% by weight of styrene, from 1 to 10% byweight of at least one monomer selected from the group consisting ofα,β-mono-ethylenically unsaturated carboxylic acids having 3 to 6 carbonatoms, amides and nitriles thereof, andfrom 0 to 10% by weight ofcrosslinking monomers, which contains at least one addedstarch-degradation product which is water-soluble at room temperatureand obtained by hydrolysis in the aqueous phase and has a weight averagemolecular weight M_(w) of from 2500 to
 25000. 2. The aqueous polymerdispersion as claimed in claim 1, obtained by carrying out thefree-radical polymerization in the presence of said starch-degradationproduct having a weight average molecular weight M_(w) of from 2500 to25000 which is obtained by hydrolysis in the aqueous phase, and by themethod of free-radical aqueous emulsion polymerization.
 3. The aqueouspolymer dispersion as claimed in claim 2, obtained by carrying out thefree-radical aqueous emulsion polymerization at a temperature from 75°to 90° C.
 4. The aqueous polymer dispersion as claimed in claim 2,obtained by carrying out the polymerization in the presence, asfree-radical polymerization initiator, of an alkali metalperoxydisulfate or ammonium peroxydisulfate or a mixture thereof.
 5. Theaqueous polymer dispersion as claimed in claim 2, obtained by carryingout the free-radical aqueous emulsion polymerization by the feedprocess, in which part of the polymerization batch containing all thestarch-degradation product to be used, some of the monomers and some ofthe initiator system in aqueous solution, is heated to thepolymerization temperature and partially polymerized, and the remainderof the polymerization batch is subsequently fed to the polymerizationzone while the polymerization is maintained.
 6. The polymer dispersionas claimed in claim 1, wherein the starch-degradation product has anM_(w) in the range from 4000 to
 16000. 7. The polymer dispersion asclaimed in claim 1, wherein the starch-degradation product has amolecular weight distribution whose nonuniformity U is in the range from6 to
 12. 8. The polymer dispersion as claimed in claim 1, wherein thestarch-degradation product has a molecular weight distribution withnonuniformity U of from 7 to
 11. 9. The polymer dispersion as claimed inclaim 1, wherein at least 10% by weight, but not more than 70% byweight, of the starch-degradation product has a molecular weight of lessthan
 1000. 10. The polymer dispersion as claimed in claim 1, wherein thestarch-degradation product has a dextrose equivalent DE of from 5 to 40.11. The polymer dispersion as claimed in claim 1, wherein thestarch-degradation product has a dynamic viscosity θ⁴⁰ (Pa.s) of from0.01 to 0.06, determined in accordance with DIN 53019, in 40% strengthby weight aqueous solution at 25° C. and a shear gradient of 75s⁻¹. 12.The polymer dispersion as claimed in claim 1, wherein thestarch-degradation product has a bimodal molecular weight distribution.13. The polymer dispersion as claimed in claim 1, wherein thestarch-degradation product has been chemically modified byetherification or esterification.
 14. The polymer dispersion as claimedin claim 1, wherein the starch-degradation product is present in anamount from 10 to 75% by weight, based on the amount of polymerizedmonomers.
 15. The polymer dispersion as claimed in claim 1, wherein thestarch-degradation product is present in an amount of from 20 to 60% byweight, based on the amount of polymerized monomers.
 16. The polymerdispersion as claimed in claim 1, wherein the starch-degradation producthas an aqueous solubility of at least 40% at 25° C.
 17. The polymerdispersion as claimed in claim 1, wherein the starch-degradation producthas an aqueous solubility above 50% at room temperature.
 18. The aqueouspolymer dispersion as claimed in claim 1, wherein the starch-degradationproduct has a bimodal molecular weight distribution, a non-uniformity Uof 6 to 12, a dextrose equivalent, DE, of from 5 to 40 and a dynamicviscosity, θ⁴⁰ (Pa.s) of from 0.01 to 0.06, determined in accordancewith DIN 53019, in 40% strength by weight aqueous solution at 25° C. anda shear gradient of 75s⁻¹, wherein at least 10%, but not more than 70%of the starch-degradation product has molecular weight below
 1000. 19.An aqueous polymer dispersion obtained by free-radical polymerization ofa monomer mixture containing from 70 to 100% by weight of styrene orbutadiene or a mixture thereof, which contains at least one addedstarch-degradation product which is water-soluble at room temperatureand obtained by hydrolysis in the aqueous phase and has a weight averagemolecular weight M_(w) of from 2500 to
 25000. 20. The aqueous polymerdispersion as claimed in claim 19, obtained by carrying out thefree-radical polymerization in the presence of said starch-degradationproduct having a weight average molecular weight M_(w) of from 2500 to25000 which is obtained by hydrolysis in the aqueous phase, and by themethod of free-radical aqueous emulsion polymerization, obtained bycarrying out the free-radical polymerization in the presence of saidstarch-degradation product and by the method of free-radical aqueousemulsion polymerization.
 21. The aqueous polymer dispersion as claimedin claim 20, obtained by carrying out the free-radical aqueous emulsionpolymerization at a temperature from 75° to 90° C.
 22. The aqueouspolymer dispersion as claimed in claim 20, obtained by carrying out thepolymerization in the presence, as free-radical polymerizationinitiator, of an alkali metal peroxydisulfate or ammoniumperoxydisulfate or a mixture thereof.
 23. The aqueous polymer dispersionas claimed in claim 20, obtained by carrying out the free-radicalaqueous emulsion polymerization by the feed process, in which part ofthe polymerization batch containing all the starch-degradation productto be used, some of the monomers and some of the initiator system inaqueous solution, is heated to the polymerization temperature andpartially polymerized, and the remainder of the polymerization batch issubsequently fed to the polymerization zone while the polymerization ismaintained.
 24. The polymer dispersion as claimed in claim 19, whereinthe starch-degradation product has an M_(w) in the range from 4000 to16000.
 25. The polymer dispersion as claimed in claim 19, wherein thestarch-degradation product has a molecular weight distribution whosenonuniformity U is in the range from 6 to
 12. 26. The polymer dispersionas claimed in claim 19, wherein the starch-degradation product has amolecular weight distribution with nonuniformity U of from 7 to
 11. 27.The polymer dispersion as claimed in claim 19, wherein at least 10% byweight, but not more than 70% by weight, of the starch-degradationproduct has a molecular weight of less than
 1000. 28. The polymerdispersion as claimed in claim 19, wherein the starch-degradationproduct has a dextrose equivalent DE of from 5 to
 40. 29. The polymerdispersion as claimed in claim 19, wherein the starch-degradationproduct has a dynamic viscosity θ⁴⁰ (Pa.s) of from 0.01 to 0.06,determined in accordance with DIN 53019, in 40% strength by weightaqueous solution at 25° C. and a shear gradient of 75s⁻¹.
 30. Thepolymer dispersion as claimed in claim 19, wherein thestarch-degradation product has a bimodal molecular weight distribution.31. The polymer dispersion as claimed in claim 19, wherein thestarch-degradation product has been chemically modified byetherification or esterification.
 32. The polymer dispersion as claimedin claim 19, wherein the starch-degradation product is present in anamount from 10 to 75% by weight, based on the amount of polymerizedmonomers.
 33. The polymer dispersion as claimed in claim 19, wherein thestarch-degradation product is present in an amount of from 20 to 60% byweight, based on the amount of polymerized monomers.
 34. The polymerdispersion as claimed in claim 19, wherein the starch-degradationproduct has an aqueous solubility of at least 40% at 25° C.
 35. Thepolymer dispersion as claimed in claim 19, wherein thestarch-degradation product has an aqueous solubility above 50% at roomtemperature.
 36. The aqueous polymer dispersion as claimed in claim 19,wherein the starch-degradation product has a bimodal molecular weightdistribution, a non-uniformity U of 6 to 12, a dextrose equivalent, DE,of from 5 to 40 and a dynamic viscosity, θ⁴⁰ (Pa.s) of from 0.01 to0.06, determined in accordance with DIN 53019, in 40% strength by weightaqueous solution at 25° C. and a shear gradient of 75s⁻¹, wherein atleast 10%, but not more than 70% of the starch-degradation product hasmolecular weight below
 1000. 37. An aqueous polymer dispersion obtainedby free-radical polymerization of a monomer mixture containing from 70to 100% by weight of vinyl chloride or vinylidene chloride or a mixturethereof, which contains at least one added starch-degradation productwhich is water-soluble at room temperature and obtained by hydrolysis inthe aqueous phase and has a weight average molecular weight M_(w) offrom 2500 to
 25000. 38. The polymer dispersion as claimed in claim 37,wherein the starch-degradation product has an aqueous solubility of atleast 40% at 25° C.
 39. The polymer dispersion as claimed in claim 37,wherein the starch-degradation product has an aqueous solubility above50% at room temperature.
 40. An aqueous polymer dispersion obtained byfree-radical polymerization of a monomer mixture containingfrom 90 to99% by weight of esters of acrylic or methacrylic acid with alkanolshaving 1 to 8 carbon atoms or mixture thereof, or styrene or a mixturethereof, and from 1 to 10% by weight of acrylic acid, methacrylic acidor a mixture thereof,which contains at least one addedstarch-degradation product which is water-soluble at room temperatureand obtained by hydrolysis in the aqueous phase and has a weight averagemolecular weight M_(w) of from 2500 to
 25000. 41. The polymer dispersionas claimed in claim 40, wherein the starch-degradation product has anaqueous solubility of at least 40% at 25° C.
 42. The polymer dispersionas claimed in claim 40, wherein the starch-degradation product has anaqueous solubility above 50% at room temperature.
 43. The aqueouspolymer dispersion as claimed in claim 40, wherein thestarch-degradation product has a bimodal molecular weight distribution,a non-uniformity U of 6 to 12, a dextrose equivalent, DE, of from 5 to40 and a dynamic viscosity, θ⁴⁰ (Pa.s) of from 0.01 to 0.06, determinedin accordance with DIN 53019, in 40% strength by weight aqueous solutionat 25° C. and a shear gradient of 75s⁻¹, wherein at least 10%, but notmore than 70% of the starch-degradation product has molecular weightbelow 1000.